1. Field of the Invention
The present invention relates to a light guiding plate, a method of manufacturing the same and a liquid crystal display (LCD) having the light guiding plate, and more particularly, to a light guiding plate having a slimmed thickness and small volume, in which a structure of a light supplying unit for supplying a light having an optical distribution of a planar light source into a LCD panel is improved to accomplish an enhancement in brightness, a low power consumption, an enhancement in optical uniformity and minimization in light loss. Further, the invention relates to a method of manufacturing the light guiding plate and a liquid crystal display having the light guiding plate.
2. Description of the Related Art
Generally, an LCD is defined as a flat panel display that allows a user to recognize data processed in an information processing unit as characters, images and moving pictures using an optical property of liquid crystal in which light transmittance is varied depending on intensity of an applied electric field.
Compared with a traditional cathode ray tube (CRT), LCDs offer various advantages such as lighter weight and smaller volume, even though the LCDs have the same resolution and screen size as the CRT.
Such an LCD includes two sheets of very thin glass substrates facing each other and a liquid crystal layer interposed between the two glass substrates. In order to provide an electric field having a variable intensity in a small area unit, there is formed a plate-shaped electrode on an inner surface of one glass substrate while there are formed plural electrodes each having an area corresponding to a desired resolution on an inner surface of the other glass substrate. After that, the two substrates are assembled with a space therebetween and liquid crystal is injected into the space in which light transmittance in the liquid crystal is varied depending on the intensity of an electric field which is applied to the liquid crystal.
The liquid crystal layer functioning to control the light transmittance does not generate the light directly. This means that the light should be supplied toward the liquid crystal for performing the displaying operation. The light can be supplied from a natural light source or an artificial light source as a whole. In case of an artificial light source, the light is generated by consuming electric energy.
The LCD using a natural light source has a fatal defect in that it is impossible to perform the displaying operation in a place where there is no natural light. Therefore, LCDs using an artificial light have been actively developed and spread.
However, there are several problems in order to obtain a high quality image using an artificial light source. As an example, it is difficult to obtain a planar light source having an optical uniformity, i.e., a uniform brightness throughout the entire display area of the LCD.
FIG. 1 is a simplified cross-sectional view of an LCD panel provided with a CCFT (Cold cathode fluorescent tube) lamp having an optical distribution of linear light source and it shows that the linear light generated from the CCFT lamp 1 is converted into a planar light having an optical distribution of a planar light source.
Such a conversion from the linear light to the planar light is achieved by a light guiding plate 3.
The light guiding plate 3 is manufactured by using a synthetic resin-based material and the light guiding plate 3 has a plate shape in a uniform thickness or a wedge shape whose thickness increases as it travels from one end to the other end.
Such a structure renders the light having an optical distribution of a linear light source and having been generated from a lamp 1 to be incident from a sidewall 3a of the light guiding plate 3. The inputted light is reflected at an inner bottom surface 3b of the light guiding plate 3 and then outputted toward a direction of “A” through an upper surface 3c of the light guiding plate 3.
The light guiding plate 3 is appropriate for the planar light source but it is inappropriate for the light efficiency. In other words, the light guiding plate 3 has a disadvantage in that the light efficiency is lowered due to a light leakage.
In order to enhance the light efficiency of the light guiding plate 3, there are formed a plurality of reflection dots 3d in an outer bottom surface of the light guiding plate 3 by a silk screen method. However, these reflection dots 3d cause other problems.
As one problem, for example, when the light guiding plate 3 having the reflection dots 3d is used for a long time, an original color of the reflection dots 3d is changed. The color-changed reflection dots 3d generates a yellowing phenomenon in which when an incident light is reflected by the reflection dots 3d, the color of the reflected light is changed into a golden yellow. The yellowing phenomenon makes it difficult to display a desired color image.
As another problem, it is difficult to form the dots 3d having a size of 100 μm or less by using the silk screen method.
As still another problem, when forming the reflection dots 3d, a failure in a desired shape occurs frequently.
As further still another problem, when the light guiding plate is used for obtaining the planar light source, an optical distribution in the configuration of the planar light source can be obtained but the brightness distribution becomes non-uniform as indicated by reference number 5 in FIG. 4.
Thus, in order to overcome the low display performance occurring when the light guiding plate 3 is used, a diffusion plate 6 is provided on the light guiding plate 3 as shown in FIG. 2. Reference symbol “B” in FIG. 2 indicates a leakage light that is leaked through a side portion of the light guiding plate 3.
The diffusion plate 6 serves to diffuse the incident light that is incident from the light guiding plate 3 such that the light irradiated from the light guiding plate 3 has an optical distribution indicated by a reference numeral 7 of FIG. 4. Further, the diffusion plate 6 functions to prevent the reflection dots 3d of the light guiding plate 3 from being visualized.
Thus, in the case that the diffusion plate 6 is used, the light irradiated from the light guiding plate 3 is scattered and thus a deficiency in the optical uniformity is somewhat overcome. However, the directionality of the light is lost during the light scattering. Therefore, after the light is incident into an LCD panel assembly 8, the amount of the light decreases or the visual angle is lowered.
In order to overcome this problem, as shown in FIG. 3, at least one prism sheet 9 is provided on the diffusion plate 6. The prism sheet 9 changes an optical distribution of the light diffused through the diffusion plate 6 into an optical distribution indicated by a reference numeral 10 in FIG. 4.
In the meantime, a part of the light that is incident into the light guiding plate 3 is reflected by the light guiding plate 3 and is directed toward the LCD panel assembly 8 but another part is leaked through the bottom surface of the light guiding plate 3 as shown in FIGS. 1 through 3. In FIG. 1, a reference symbol “C” indicates a leakage light leaked toward the outside.
Thus, if a part of the light is leaked through the bottom surface of the light guiding plate 3, a light directed toward the LCD panel assembly 8 is insufficient naturally, so that an amount of the brightness that is necessary to display an image definitely is lacking.
In order to overcome this problem, as shown in FIG. 2, there is further provided a reflection plate 11 for reproducing the leakage light leaked from the light guiding plate 3 while having a high reflectivity below the light guiding plate 3.
Thus, in the case that the light guiding plate and the reflection dots 3d are used, the aforementioned diffusion plate 6, the prism sheet 9, the reflection plate 11 and the like are additionally needed, so that the production costs increase substantially. The diffusion plate 6, the prism sheet 9 and the reflection plate 11 are stacked and received in the named order in a receiving container (not shown), to thereby compose a so-called backlight assembly.
While the aforementioned diffusion plate 6, the prism sheet 9 and the reflection plate 11 enhance the optical uniformity of the light, a considerable amount of the light generated from the lamp 1 such as a CCFT lamp is consumed.
Thus, in order to obtain a sufficient brightness necessary for the displaying of an image in the LCD, the amount of the light generated from the CCFT lamp 1 should increase considering a lost amount of the light in the backlight assembly. This means a substantial increase in power consumption.
Furthermore, the diffusion plate 6, the prism sheet 9 and the reflection sheet 11 act to increase the thickness of the LCD, as well as time spent in the assembly of the LCD and its production costs.